Radzinsky's biography of the "mad monk" Rasputin begins with a cast list, a necessary point of reference in this detailed re-examination of the infamous anti-hero of the last days of the Russian tsars. The names are familiar - Nicholas and Alexandra, the last Romanovs, their haemophiliac son Alexei (or "Sunbeam", as his doting mother called him) and, of course, Grigory Rasputin, or Grisha to his friends.

As the much picked-over tale of the fall of the Romanovs has it, the licentious Rasputin used his uncanny powers to staunch young Alexei's bleeding to inveigle his way to the throne. Alexandra's belief in him and her domination of her weak-willed husband meant that she and her lover became the rulers of the Russian Empire in its last chaotic days. By the time his enemies had finally poisoned, shot and thrown this still struggling diabolic creature into the icy waters of Petrograd, it was too late. In the words of Kerensky, prime minister after the February Revolution: "Without Rasputin, there would have been no Lenin."

As in all legends, there is truth mixed in with melodrama, and even sober histories of the Russian revolutions cannot dismiss as fanciful gossip the fantastical stories of Rasputin's debauchery and drinking. The Siberian peasant with the hypnotic gaze attracted a gaggle of ladies who showed their devotion by sewing his nail clippings to their underwear. When Nicholas was away presiding over the Russian army's disastrous campaigns, he and the tsarina conspired to replace government ministers with people willing to share Alexandra's belief that her trusted "holy fool" was an envoy from God.

However, as Radzinsky demonstrates, Rasputin was more complex and more substantial than the myths that grew up around him. To understand how a semi-literate peasant came to be vilified as the satanic force respon sible for ending 300 years of Romanov rule, one must look beyond his relationship with the "Mama and Papa" of the Russian people and examine the complex loyalties and animosities of the grand dukes and duchesses, the role of secret policemen and clerics as well as the nature of Russian spirituality.

The "mad monk" of pop history was never a monk, nor was he in any medical sense mad. His was a peculiar, but not uncommon, religious fervour with deep historical roots in peasant distrust of official Russian Orthodoxy and a mystical belief in the ruler's bonds with the common people. Born into a peasant family less than a decade after Alexander II had abolished serfdom, he was known to fellow villagers as "Grisha the fool" until, at the age of 28, he left his home and began wandering from monastery to monastery in search of spiritual enlightenment.

Rasputin's quest was not unusual; nor was the fact that he was drawn to sectarianism, which had, by the end of the 19th century, become all-pervasive. He became involved with the Khlysti, a sect that believed they could become closer to God through group sex; in 1903 he was denounced, but had already attracted powerful devotees who protected him from persecution. Escaping official condemnation, this "man of God" made his way to St Petersburg and, by the end of the year, was being received in the houses of the tsar's relatives.

Before Rasputin, Alexandra had received "Mitya the nasal voiced" and "Matryona the barefoot"; Rasputin was merely the last and most favoured of a quartet of "holy fools" or "elders" who provided spiritual sustenance and a supposed link to the common people.

Radzinsky centres his biography on a newly discovered file, a record of an investigation into Rasputin which contains testimony gathered by a government commission in March 1917. Radzinsky believes it to be the missing piece of the historical jigsaw that allows him to write "the last word" on Rasputin. It contains reports from secret policemen, depositions by monks who met him during his wanderings and testimony from the coterie of high-born St Petersburg women who became Rasputin's apostles and probably his lovers.

One of Russia's best-known dramatists, Radzinsky recounts Rasputin's story with an unerring sense of the theatrical and an atmospheric evocation of the court. His conversational, occasionally florid style might irritate those seeking a scholarly tome (as does the lack of footnotes and the absence of attribution for quotes), but this is an immensely readable and gripping bedside history.

The author of hugely successful biographies of Nicholas and Alexandra and Stalin, Radzinsky knows the period well; he delves far deeper into the enigma of Rasputin than his predecessors, and provides new evidence on how Rasputin met his death. Holy man or sinner? A cause of the revolution or merely a symptom of the rot? For Radzinsky, Rasputin is all of these, as well as a uniquely Russian manifestation of religious belief.